1. Field of the Invention
The present invention relates generally to adjusting one or more operating parameters for toner transfer in a direct transfer image forming apparatus and, more particularly, to methods of transfer voltage controls to prevent print defects.
2. Description of the Related Art
Certain image forming devices use an electrographic process to develop toner images on a media sheet. The electrophotographic process uses electrostatic voltage differentials to promote the transfer of toner from component to component. For example, a voltage vector may exist between a developer roll and a latent image on a photoconductive member. This voltage vector helps promote the transfer of toner from the developer roll to the latent image in a process that is sometimes called “developing the image.” A separate voltage vector may exist within a transfer nip formed between the photoconductive member and a transfer member to promote the transfer of a developed image onto a media sheet. In each instance, the toner transfer occurs in part because the toner itself is charged and is attracted to surfaces having an opposite charge or a lower potential.
In a direct transfer system where toner is moved directly from the photoconductive member to the media sheet, current flow between the transfer member and the photoconductive member may produce an undesirable charge on the photoconductive member. A non-uniform current may be produced on the photoconductive member when a leading edge of the media sheet enters into the transfer nip formed between the photoconductive member and the transfer member. The entering media sheet causes a large negative spike in the current that occurs because the current path between the photoconductive member and the transfer member is momentarily disrupted. A non-uniform current may also be produced when the trailing edge of the media sheet exits the transfer nip. The exiting media sheet causes a large negative spike that occurs because the current path between the photoconductive member and transfer member is momentarily disrupted. Once the media sheet exits the transfer nip, contact with the photoconductive member is reestablished and a large positive current spike occurs due to the excess charge that has built up and is released.
The current should be controlled with excessive spikes in the positive or negative direction limited to prevent the occurrence of print defects. If not controlled, a negative spike in the transfer current may result as a light band due to a relative over-charging of the photoconductive member. A positive spike may appear as a dark band where the photoconductive member is discharged and cannot be fully recharged.
Previously, the large transfer current spikes caused by the media sheet entering and exiting the transfer nip have been offset by using a ramped transfer voltage including a series of alternating positive and negative steps that generally trend to ramp up or down. A common drawback of this approach is when this technique is applied in a humid environment, the amplitude of the current oscillations grows too large, resulting in a new print defect. Thus, there is still a need for an innovation that will adjust the voltage waveform oscillations in response to temperature and humidity environmental conditions in order to maintain a uniform charge on the surface of the photoconductor.